We present the first volume-limited sample of cataclysmic variables (CVs), selected using the accurate parallaxes provided by the second data release (DR2) of the European Space Agency Gaia space mission. The sample is composed of 42 CVs within 150 pc, including two new systems discovered using the Gaia data, and is $(77 \pm 10)$ per cent complete. We use this sample to study the intrinsic properties of the Galactic CV population. In particular, the CV space density we derive, $\rho =(4.8^{+0.6}_{-0.8}) \times 10^{-6}\, \mbox{$\mathrm{pc}^{-3}$}$, is lower than that predicted by most binary population synthesis studies. We also find a low fraction of period bounce CVs, seven per cent, and an average white dwarf mass of $\langle M_\mathrm{WD} \rangle = (0.83 \pm 0.17)\, \mathrm{M}_\odot$. Both findings confirm previous results, ruling out the presence of observational biases affecting these measurements, as has been suggested in the past. The observed fraction of period bounce CVs falls well below theoretical predictions, by at least a factor of five, and remains one of the open problems in the current understanding of CV evolution. Conversely, the average white dwarf mass supports the presence of additional mechanisms of angular momentum loss that have been accounted for in the latest evolutionary models. The fraction of magnetic CVs in the 150 pc sample is remarkably high at 36 per cent. This is in striking contrast with the absence of magnetic white dwarfs in the detached population of CV progenitors, and underlines that the evolution of magnetic systems has to be included in the next generation of population models.

We present measurements of the periods of dwarf nova oscillations (DNOs) and quasi‐periodic oscillations (QPOs) in cataclysmic variable stars (CVs), many culled from published literature, but also others newly observed (in VZ Pyx, CR Boo, OY Car, Z Cha, AQ Eri, TU Men, HX Peg, CN Ori, V893 Sco, WX Hyi and EC2117−54). These provide data for 26 systems. We show that in general PQPO∼ 15 PDNO and that the correlation for CVs extends by three orders of magnitude lower in frequency the similar relationship found for X‐ray binaries. In addition, we have found that there is a second type of DNO, previously overlooked, which have periods ∼4 times those of the regular DNOs (as well as those mined from publications, we have observed them in VW Hyi, OY Car, AQ Eri, V803 Cen, CR Boo, VZ Pyx, HX Peg and EC2117−54). Often both types of DNO coexist. Unlike the standard DNOs, the periods of the new type, which we refer to as longer‐period DNOs (lpDNOs), are relatively insensitive to accretion luminosity and can even appear in quiescence of dwarf novae. We interpret them as magnetically channelled accretion on to the differentially rotating main body of the white dwarf primary, rather than on to a rapidly slipping equatorial belt as in the case of the standard DNOs. This is supported by published measurements of v sin i for some of the primaries. Some similarities of the DNOs, lpDNOs and QPOs in CVs to the three types of QPO in X‐ray binaries (burst pulsations, high‐ and low‐frequency QPOs) are noted.

We here report on the multiwavelength study which led us to the identification of X-ray source IGR J16194−2810 as a new Symbiotic X-ray Binary (SyXB), that is, a rare type of Low Mass X-ray Binary (LMXB) composed of a M-type giant and a compact object. Using the accurate X-ray position allowed by Swift/XRT data, we pinpointed the optical counterpart, a M2 III star. Besides, the combined use of the spectral information afforded by XRT and INTEGRAL/IBIS shows that the 0.5−200 keV spectrum of this source can be described with an absorbed Comptonization model, usually found in LMXBs and, in particular, in SyXBs. No long-term (days to months) periodicities are detected in the IBIS data. The time coverage afforded by XRT reveals shot-noise variability typical of accreting Galactic X-ray sources, but is not good enough to explore the presence of X-ray short-term (seconds to hours) oscillations in detail. By using the above information, we infer important parameters for this source such as its distance (∼3.7 kpc) and X-ray luminosity (∼1.4 × 10 35 erg s −1 in the 0.5−200 keV band), and we give a description for this system (typical of SyXBs) in which a compact object (possibly a neutron star) accretes from the wind of its M-type giant companion. We also draw some comparisons between IGR J16194−2810 and other sources belonging to this subclass, finding that this object resembles SyXBs 4U 1700+24 and 4U 1954+31.

We combine two complete, X‐ray flux‐limited surveys, the ROSAT Bright Survey (RBS) and the ROSAT North Ecliptic Pole (NEP) survey, to measure the space density (ρ) and X‐ray luminosity function (Φ) of non‐magnetic cataclysmic variables (CVs). The combined survey has a flux limit of FX≳ 1.1 × 10−12 erg cm−2 s−1 over most of its solid angle of just over , but is as deep as ≃10−14 erg cm−2 s−1 over a small area. The CV sample that we construct from these two surveys contains 20 non‐magnetic systems. We carefully include all sources of statistical error in calculating ρ and Φ by using Monte Carlo simulations; the most important uncertainty proves to be the often large errors in distances estimates. If we assume that the 20 CVs in the combined RBS and NEP survey sample are representative of the intrinsic population, the space density of non‐magnetic CVs is . We discuss the difficulty in measuring Φ in some detail – in order to account for biases in the measurement, we have to adopt a functional form for Φ. Assuming that the X‐ray luminosity function of non‐magnetic CVs is a truncated power law, we constrain the power‐law index to −0.80 ± 0.05. It seems likely that the two surveys have failed to detect a large, faint population of short‐period CVs, and that the true space density may well be a factor of 2 or 3 larger than what we have measured; this is possible, even if we only allow for undetected CVs to have X‐ray luminosities in the narrow range 28.7 < log(LX/erg s−1) < 29.7. However, ρ as high as 2 × 10−4 pc−3 would require that the majority of CVs has X‐ray luminosities below LX= 4 × 1028 erg s−1 in the 0.5–2.0 keV band.

We use the ROSAT North Ecliptic Pole (NEP) survey to construct a small, but purely X‐ray flux‐limited sample of cataclysmic variable stars (CVs). The sample includes only four systems, two of which (RX J1715.6+6856 and RX J1831.7+6511) are new discoveries. We present time‐resolved spectroscopy of the new CVs and measure orbital periods of 1.64 ± 0.02 and 4.01 ± 0.03 h for RX J1715.6+6856 and RX J1831.7+6511, respectively. We also estimate distances for all the CVs in our sample, based mainly on their apparent brightness in the infrared. The space density of the CV population represented by our small sample is 1.1+2.3−0.7× 10−5 pc−3. We can also place upper limits on the space density of any subpopulation of CVs too faint to be included in the NEP survey. In particular, we show that if the overall space density of CVs is as high as 2 × 10−4 pc−3 (as has been predicted theoretically), the vast majority of CVs must be fainter than LX≃ 2 × 1029 erg s−1.

Large differences between the properties of the known sample of cataclysmic variable stars (CVs) and the predictions of the theory of binary star evolution have long been recognized. However, because all existing CV samples suffer from strong selection effects, observational biases must be taken into account before it is possible to tell whether there is an inconsistency. In order to address this problem, we have modelled the impact of selection effects on observed CV samples using a Monte Carlo approach. By simulating the selection criteria of the Palomar–Green (PG) Survey, we show that selection effects cannot reconcile the predictions of standard CV evolution theory with the observed sample. More generally, we illustrate the effect of the biases that are introduced by magnitude limits, selection cuts in U−B and restrictions in Galactic latitude.

We use the complete, X-ray flux-limited ROSAT Bright Survey (RBS) to measure the space density (ρ) of magnetic cataclysmic variables (mCVs). The survey provides complete optical identification of all sources with count rate > 0.2 s −1 (corresponding to F X 2 × 10 −12 erg cm −2 s −1 ) over half the sky (|b| > 30 • ), and detected 6 intermediate polars (IPs) and 24 polars. If we assume that the 30 mCVs included in the RBS are representative of the intrinsic population, the space density of mCVs is 8 +4 −2 × 10 −7 pc −3 . Considering polars and IPs separately, we find ρ polar = 5 +3 −2 × 10 −7 pc −3 and ρ IP = 3 +2 −1 × 10 −7 pc −3 . Allowing for a 50% high-state duty cycle for polars (and assuming that these systems are below the RBS detection limit during their low states) doubles our estimate of ρ polar and brings the total space density of mCVs to 1.3 +0.6 −0.4 × 10 −6 pc −3 . We also place upper limits on the sizes of faint (but persistent) mCV populations that might have escaped detection in the RBS. Although the large uncertainties in the ρ estimates prevent us from drawing strong conclusions, we discuss the implications of our results for the evolutionary relationship between IPs and polars, the fraction of CVs with strongly magnetic white dwarfs (WDs), and for the contribution of mCVs to Galactic populations of hard Xray sources at L X 10 31 ergs −1 . Our space density estimates are consistent with the very simple model where long-period IPs evolve into polars and account for the whole short-period polar population. We find that the fraction of WDs that are strongly magnetic is not significantly higher for CV primaries than for isolated WDs. Finally, the space density of IPs is sufficiently high to explain the bright, hard X-ray source population in the Galactic Centre.

The European Space Agency γ‐ray telescope, INTEGRAL, is detecting relatively more intrinsically rare cataclysmic variables (CVs) than were found by surveys at lower energies. Specifically, a large fraction of the CVs that are INTEGRAL sources consists of asynchronous polars and intermediate polars (IPs). IP classifications have been proposed for the majority of CVs discovered by INTEGRAL, but, in many cases, there is very little known about these systems. In order to address this, I present time‐resolved optical data of five CVs discovered through INTEGRAL observations. The white dwarf spin modulation is detected in high‐speed photometry of three of the new CVs (IGR J15094−6649, IGR J16500−3307 and IGR J17195−4100), but two others (XSS J12270−4859 and IGR J16167−4957) show no evidence of magnetism, and should be considered unclassified systems. Spectroscopic orbital period (Porb) measurements are also given for IGR J15094−6649, IGR J16167−4957, IGR J16500−3307 and IGR J17195−4100.