Interacting binaries W Serpentids and Double Periodic Variables

W Serpentids and Double Periodic Variables (DPVs) are candidates for close interacting binaries in a non-conservative evolutionary stage; while W Serpentids are defined by high-excitation ultraviolet emission lines present during most orbital phases, and by usually showing variable orbital periods, DPVs are characterized by a long photometric cycle lasting roughly 33 times the (practically constant) orbital period. We report the discovery of 7 new Galactic DPVs, increasing the number of known DPVs in our Galaxy by 50%. We find that DPVs are tangential-impact systems, i.e. their primaries have radii barely larger than the critical Lubow-Shu radius. These systems are expected to show transient discs, but we find that they host stable discs with radii smaller than the tidal radius. Among tangential-impact systems including DPVs and semi-detached Algols, only DPVs have primaries with masses between 7 and 10 $M_{\odot}$. We find that DPVs are in a Case-B mass transfer stage with donor masses between 1 and 2 M$_{\odot}$ and with primaries resembling Be stars. W Serpentids are impact and non-impact systems, their discs extend until the last non-intersecting orbit and show a larger range of stellar mass and mass ratio than DPVs. Infrared photometry reveals significant color excesses in many DPVs and W Serpentids, usually larger for the latter ones, suggesting variable amounts of circumstellar matter.