2002MNRAS.335..593B

We present high-resolution echelle spectroscopy that confirms, via the identification of numerous chromospheric emission lines, the extremely high levels of chromospheric activity displayed by the secondary stars in post-common-envelope binaries (PCEBs). Furthermore, the EG UMa data set includes the first ever high-resolution spectroscopic observations of flare activity in a PCEB system. The nature of the EG UMa spectra is investigated and a two-flare model that explains all of the observed emission-line features is identified. V -band photometry of EG UMa includes a further flare event, but contains no evidence of pulsations from the white dwarf primary star in this system.

The extensive spectral-line emission observed in EG UMa is also observed in HZ 9. The criteria used to identify flare events in the EG UMa data is employed to classify the HZ 9 spectrum as an observation of extremely high level, quiescent, chromospheric activity, and not arising from a flare event. PG 1026+002 and RE J1629+780 both display H Balmer and He i D₃line emission. The observed activity of the secondary star in PG 1026+002 appears, however, to be at a lower level than that exhibited by the secondary stars in EG UMa, HZ 9 and RE J1629+780. It is not clear why PG 1026+002 should be exhibiting a lower level of activity, nor is it possible to deduce whether this is a transient phenomenon. The only spectral features attributable to the secondary star in GD 123 are weak Hα line emission and Na i doublet line absorption at 8183, 8195 Å.

The He i D₃line is a magnetic activity indicator and its presence in all of the PCEB spectra, with exception of GD 123, is direct evidence for the maintenance of magnetic fields in all of these late-type, fully convective, dwarf secondary stars.