A NEW CLASS OF RAPIDLY PULSATING STAR .4. OSCILLATIONS IN EC-20117-4014 AND ATMOSPHERIC ANALYSES

A new class of pulsating stars, the EC 14026 stars, has recently been announced. We report the discovery of rapid oscillations in EC 20117 - 4014. The oscillations are multiperiodic with three periods detected so far: 137.3, 142.1 and 158.7 s. The peak-to-peak amplitude of these oscillations is similar to 1 per cent. Along with the spectrum and UBV colours, this implies that EC 20117 - 4014 is another member of the n ew class. Interestingly, there is evidence for period/phase changes in the main 137.3-s period, as no linear ephemeris can be found that wil l fit the collection of light curves we have obtained. We interpret th e oscillations as pulsations of an sdB star in a binary with an F/G co mpanion, as indicated by the optical spectra. In order to derive atmos pheric parameters of the component stars, low- (similar to 3.5 Angstro m) and intermediate- (similar to 1 Angstrom) resolution optical spectr oscopy is presented for the first four EC14026 stars to be discovered: EC14026 - 2647, EC10228 - 0905, EC 20117 - 4014 and PB 8783. Their sp ectra are modelled as the combination of sdB and FIG stars using a gri d of high-gravity model atmospheres to represent the former, and obser ved spectra to represent the latter. The spectral types of the late-ty pe companions are estimated to be in the range early F to early G. Atm ospheric parameters for the sdB components of the four stars are deriv ed and found to be similar: log g similar to 6.0 and T-eff similar to 35 000 K. If the late-type companions are on or near the main sequence , their distances agree with those estimated for the sdB components, a ssuming that the masses of the latter are similar to 0.5 M.. The evolu tionary status of the four stars strongly suggests that the sdB compon ent has suffered substantial mass loss by Roche lobe overflow, along t he lines originally proposed by Mengel et al. This hypothesis can be t ested by using the orbital time delay in the pulsations to measure the binary period and separation. The discovery of pulsations in sdB star s is important: asteroseismological studies to probe their structure a nd evolutionary time-scale can now be contemplated.