  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.
