Dwarf novae in the Hamburg quasar survey: rarer than expected

Aims.We report the discovery of five new dwarf novae that were spectroscopically identified in the Hamburg Quasar Survey (HQS), and discuss the properties of the sample of new dwarf novae from the HQS.
Methods.Follow-up time-resolved spectroscopy and photometry have been obtained to characterise the new systems.
Results.The orbital periods determined from analyses of the radial velocity variations and/or orbital photometric variability are P_orb≃105.1 min or P_orb≃109.9 min for HS 0417+7445, P_orb=114.3±2.7 min for HS 1016+3412, P_orb=92.66±0.17 min for HS 1340+1524, P_orb272.317±0.001 min for HS 1857+7127, and P_orb=258.02±0.56 min for HS 2214+2845. HS 1857+7127 is found to be partially eclipsing. In HS 2214+2845 the secondary star of spectral type M3±1 is clearly detected, and we estimate the distance to the system to be d=390±40 pc. We recorded one superoutburst of HS 0417+7445, identifying the system as a SU UMa-type dwarf nova. HS 1016+3412 and HS 1340+1524 have rare outbursts, and their subtype is yet undetermined. HS 1857+7127 frequently varies in brightness and may be a Z Cam-type dwarf nova. HS 2214+2845 is a U Gem-type dwarf nova with a most likely cycle length of 71 d.
Conclusions.To date, 14 new dwarf novae have been identified in the HQS. The ratio of short-period (<3 h) to long-period (>3 h) systems of this sample is 1.3, much smaller compared to the ratio of 2.7 found for all known dwarf novae. The HQS dwarf novae display typically infrequent or low-amplitude outburst activity, underlining the strength of spectroscopic selection in identifying new CVs independently of their variability. The spectroscopic properties of short-period CVs in the HQS, newly identified and previously known, suggest that most, or possibly all of them are still evolving towards the minimum period. Their total number agrees with the predictions of population models within an order of magnitude. However, the bulk of all CVs is predicted to have evolved past the minimum period, and those systems remain unidentified. This suggests that those post-bounce systems have markedly weaker Hβ emission lines compared to the average known short-period CVs, and undergo no or extremely rare outbursts.