We present time‐series optical photometry of five new cataclysmic variables (CVs) identified by the Hamburg Quasar Survey (HQS). The deep eclipses observed in HS 0129+2933 (= TT Tri), HS 0220+0603 and HS 0455+8315 provided very accurate orbital periods of 3.35129827(65), 3.58098501(34) and 3.56937674(26) h, respectively. HS 0805+3822 shows grazing eclipses and has a likely orbital period of 3.2169(2) h. Time‐resolved optical spectroscopy of the new CVs (with the exception of HS 0805+3822) is also presented. Radial velocity studies of the Balmer emission lines provided an orbital period of 3.55 h for HS 1813+6122, which allowed us to identify the observed photometric signal at 3.39 h as a negative superhump wave. The spectroscopic behaviour exhibited by all the systems clearly identifies them as new SW Sextantis (SW Sex) stars. HS 0220+0603 shows unusual N ii and Si ii emission lines suggesting that the donor star may have experienced nuclear evolution via the CNO cycle. These five new additions to the class increase the number of known SW Sex stars to 35. Almost 40 per cent of the total SW Sex population do not show eclipses, invalidating the requirement of eclipses as a defining characteristic of the class and the models based on a high orbital inclination geometry alone. On the other hand, as more SW Sex stars are identified, the predominance of orbital periods in the narrow 3–4.5 h range is becoming more pronounced. In fact, almost half the CVs which populate the 3–4.5 h period interval are definite members of the class. The dominance of SW Sex stars is even stronger in the 2–3 h period gap, where they make up 55 per cent of all known gap CVs. These statistics are confirmed by our results from the HQS CVs. Remarkably, 54 per cent of the Hamburg nova‐like variables have been identified as SW Sex stars with orbital periods in the 3–4.5 h range. The observation of this pile‐up of systems close to the upper boundary of the period gap is difficult to reconcile with the standard theory of CV evolution, as the SW Sex stars are believed to have the highest mass‐transfer rates among CVs. Finally, we review the full range of common properties that the SW Sex stars exhibit. Only a comprehensive study of this rich phenomenology will prompt to a full understanding of the phenomenon and its impact on the evolution of CVs and the accretion processes in compact binaries in general.

We report the first results from a new search for cataclysmic variables (CVs) using a combined X‐ray (ROSAT)/infrared (2MASS) target selection that discriminates against background active galactic nuclei. Identification spectra were obtained at the Isaac Newton Telescope for a total of 174 targets, leading to the discovery of 12 new CVs. Initially devised to find short‐period low‐mass‐transfer CVs, this selection scheme has been very successful in identifying new intermediate polars. Photometric and spectroscopic follow‐up observations identify four of the new CVs as intermediate polars: 1RXS J063631.9+353537 (Porb≃ 201 min, Pspin= 1008.3408 s or 930.5829 s), 1RXS J070407.9+262501 (Porb≃ 250 min, Pspin= 480.708 s), 1RXS J173021.5–055933 (Porb= 925.27 min, Pspin= 128.0 s), and 1RXS J180340.0+401214 (Porb= 160.21 min, Pspin= 1520.51 s). RX J1730, also a moderately bright hard X‐ray source in the INTEGRAL/IBIS Galactic plane survey, resembles the enigmatic AE Aqr. It is likely that its white dwarf is not rotating at the spin equilibrium period, and the system may represent a short‐lived phase in CV evolution.

Aims. We report identification of cyclical changes in the orbital period of the eclipsing cataclysmic variable HT Cas. Methods. We measured new white-dwarf mid-eclipse timings and combined them with published measurements to construct an observed-minus-calculated diagram covering 29 years of observations. Results. The data present a 36 yr period modulation of semi-amplitude ∼40 s, with a statistical significance greater than 99.9 percent with respect to a constant period. Conclusions. We combine our results with those in the literature to revisit the issue of cyclical period changes in cataclysmic variables and their interpretation in terms of a solar-type magnetic activity cycle in the secondary star. A diagram of fractional period change (∆P/P) versus the angular velocity of the active star (Ω) for cataclysmic variables, RS CVn, W UMa, and Algols, reveal that close binaries with periods above the gap (secondaries with convective envelopes) satisfy a relationship ∆P/P ∝ Ω −0.7±0.1 . Cataclysmic variables below the period gap (with fully convective secondaries) deviate from this relationship by more than 3-σ, with average fractional period changes 6 times smaller than those of the systems above the gap.

Aims. We report the discovery of a new white dwarf/M dwarf binary, HS 1857+5144, identified in the Hamburg Quasar Survey (HQS). Methods. Time-resolved optical spectroscopy and photometry were carried out to determine the properties of this new cataclysmic variable progenitor (pre-CV). Results. The light curves of HS 1857+5144 display a sinusoidal variation with a period of P orb = 383.52 min and peak-to-peak amplitudes of 0.7 mag and 1.1 mag in the B-band and R-band, respectively. The large amplitude of the brightness variation results from a reflection effect on the heated inner hemisphere of the companion star, suggesting a very high temperature of the white dwarf. Our radial velocity study confirms the photometric period as the orbital period of the system. A model atmosphere fit to the spectrum of the white dwarf obtained at minimum light provides limits to its mass and temperature of M wd 0.6−1.0 M and T wd 70 000−100 000 K, respectively. The detection of He II λ4686 absorption classifies the primary star of HS 1857+5144 as a DAO white dwarf. Combining the results from our spectroscopy and photometry, we estimate the mass of the companion star and the binary inclination to be M sec 0.15−0.30 M and i 45• −55 • , respectively. Conclusions. We classify HS 1857+5144 as one of the youngest pre-CV known to date. The cooling age of the white dwarf suggests that the present system has just emerged from a common envelope phase ∼10 5 yr ago. HS 1857+5144 will start mass transfer within or below the 2-3 h period gap.