We discuss the properties of 137 cataclysmic variables (CVs) which are included in the Sloan Digital Sky Survey (SDSS) spectroscopic data base, and for which accurate orbital periods have been measured. 92 of these systems are new discoveries from SDSS and were followed‐up in more detail over the past few years. 45 systems were previously identified as CVs because of the detection of optical outbursts and/or X‐ray emission, and subsequently re‐identified from the SDSS spectroscopy. The period distribution of the SDSS CVs differs dramatically from that of all the previously known CVs, in particular it contains a significant accumulation of systems in the orbital period range 80–86 min. We identify this feature as the elusive ‘period minimum spike’ predicted by CV population models, which resolves a long‐standing discrepancy between compact binary evolution theory and observations. We show that this spike is almost entirely due to the large number of CVs with very low accretion activity identified by SDSS. The optical spectra of these systems are dominated by emission from the white dwarf photosphere, and display little or no spectroscopic signature from the donor stars, suggesting very low mass companion stars. We determine the average absolute magnitude of these low‐luminosity CVs at the period minimum to be 〈Mg〉= 11.6 ± 0.7. Comparison of the SDSS CV sample to the CVs found in the Hamburg Quasar Survey and the Palomar Green Survey suggests that the depth of SDSS is the key ingredient resulting in the discovery of a large number of intrinsically faint short‐period systems.

Aims. During the last ∼50 years, the population of black hole candidates in X-ray binaries has increased considerably, with 59 Galactic objects being detected in transient low-mass X-ray binaries, as well as a few in persistent systems (including ∼5 extragalactic binaries). Methods. We collect near-infrared, optical, and X-ray information spread over hundreds of references to study the population of black holes in X-ray transients as a whole.Results. We present the most updated catalogue of black hole transients. This contains X-ray, optical, and near-infrared observations, together with their astrometric and dynamical properties. The catalogue provides new and useful information in both statistical and observational parameters and provides a thorough and complete overview of the black hole population in the Milky Way. Analysing the distances and spatial distribution of the observed systems, we estimate a total population of ∼1300 Galactic black hole transients. This means that we have only discovered less than ∼5% of the total Galactic distribution.

We study the K-correction for the case of emission lines formed in the X-ray-illuminated atmosphere of a Roche lobe-filling star. We compute the K-correction as a function of the mass ratio q and the disk flaring angle using a compact binary code in which the companion's Roche lobe is divided into 10 5 resolution elements. We also study the effect of the inclination angle in the results. We apply our model to the case of the neutron star low-mass X-ray binary X1822À371 (V691 CrA), where a K-emission velocity K em ¼ 300 AE 8 km s À1 has been measured by Casares et al. Our numerical results, combined with a previous determination of system parameters, yields 1:61 M M NS 2:32 M and 0:44 M M 2 0:56 M for the two binary components (i.e., 0:24 q 0:27), which provide compelling evidence for a massive neutron star in this system. We also discuss the implications of these masses into the evolutionary history of the binary.