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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.

We present time-resolved optical spectroscopy and photometry of four relatively bright (V ∼ 14.0−15.5) long-period cataclysmic variables (CVs) discovered in the Hamburg Quasar Survey: HS 0139+0559, HS 0229+8016, HS 0506+7725, and HS 0642+5049. Their respective orbital periods, 243.69 ± 0.49 min, 232.550 ± 0.049 min, 212.7 ± 0.2 min, and 225.90 ± 0.23 min are determined from radial velocity and photometric variability studies. HS 0506+7725 is characterised by strong Balmer and He emission lines, short-period (∼10−20 min) flickering, and weak X-ray emission in the ROSAT All Sky Survey. The detection of a deep low state (B 18.5) identifies HS 0506+7725 as a member of the VY Scl stars. HS 0139+0559, HS 0229+8016, and HS 0642+5049 display thick-disc like spectra and no or only weak flickering activity. HS 0139+0559 and HS 0229+8016 exhibit clean quasi-sinusoidal radial velocity variations of their emission lines but no or very little orbital photometric variability. In contrast, we detect no radial velocity variation in HS 0642+5049 but a noticeable orbital brightness variation. We identify all three systems either as UX UMa-type novalike variables or as Z Cam-type dwarf novae. Our identification of these four new systems underlines that the currently known sample of CVs is rather incomplete even for bright objects. The four new systems add to the clustering of orbital periods in the 3−4 h range found in the sample of HQS selected CVs, and we discuss the large incidence of magnetic CVs and VY Scl/SW Sex stars found in this period range among the known population of CVs. IntroductionStandard models for the population of cataclysmic variables (CVs) predict that the vast majority of all CVs should have Based on observations obtained at the German-Spanish Astronomical Center, Calar Alto, operated by the Max-PlanckInstitut für Astronomie, Heidelberg, jointly with the Spanish National Commission for Astronomy, on observations made at the 1.2 m telescope, located at Kryoneri Korinthias, and owned by the National Observatory of Athens, Greece, and on observations made with the OGS telescope, operated on the island of Tenerife by the European Space Agency, in the Spanish Observatorio del Teide of the IAC. Table 1 is only available in electronic form at http://www.edpsciences.org short orbital periods, P orb < 2 h (e.g. Kolb 1993;Howell et al. 1997), and a space density of 2 × 10 −5 −2 × 10 −4 pc −3 (Ritter & Burkert 1986;de Kool 1992;Politano 1996). These predictions contrast with the properties of the observed population of galactic CVs, with an estimated space density of ∼6 × 10 −6 pc −3 (Ringwald 1996;Araujo-Betancor et al. 2005b) and an apparent lack of short-period systems. Possible reasons for these discrepancies are uncertainties in the theory of CV evolution (e.g. King 1988;Schenker & King 2002;Andronov et al. 2003;Barker & Kolb 2003;Taam et al. 2003), but also observational selection effects in the known CV population (e.g Downes 1986;Ringwald 1996;Gänsicke 2005).About 75% of all known CVs have been discove...