Abstract

Aims. In order to better understand the double periodic variable (DPV) phenomenon, we analyse a series of optical spectra of the DPV system DQ Velorum during much of its long-term cycle. In addition, we investigate the evolutionary history of DQ Vel using theoretical evolutionary models to obtain the best representation for the current observed stellar and orbital parameters of the binary. We investigate the evolution of DQ Vel through theoretical evolutionary models to estimate the age and the mass transfer rate which are then compared with those of its twin V393 Scorpii. Methods, We subtracted the donor star contribution from the composite spectra of DQ Vel using a synthetic spectrum as a donor template. Donor subtracted spectra covering around 60% of the long-term cycle allowed us to investigate time modulated spectral variations of the gainer star plus the disc, We used Gaussian fits to measure the equivalent widths (EWs) of Balmer and helium lines in the separated spectra during the long-term cycle and thus analyse LW variabilities. We compared the observed stellar parameters of the system with a grid of theoretical evolutionary tracks computed under a conservative and a non-conservative evolution regime. Results. We have found that the EW of Balmer and helium lines in the donor-subtracted spectra are modulated with the long-term cycle. We observe a strengthening in the EWs in all analysed spectral features at the minimum of the long-term cycle which might be related to an extra line emission during the maximum of the long-term variability. Difference spectra obtained at the secondary eclipse support this scenario, We have found that a non-conservative evolutionary model where DQ Vet has lost mass at some stage of its binary history, is a better representation of the current observed properties of the system. The best evolutionary model suggests that DQ Vel has an age of 7.40 x 10(7) yr and is currently in a low mass transfer rate (-9.8 x 10(-9) M-circle dot/yr) stage, after a mass transfer burst episode. Comparing the evolutionary stages of DQ Vel and V393 Sco we observed that the former is an older system with a lower mass transfer rate. This might explain the differences observed in the physical parameters of their accretion discs.