Multicolour high-speed photometry of pulsating subdwarf B stars with ULTRACAM

High-speed multicolour photometry is presented for two pulsating subdwarf B stars, KPD 2109+4401 and HS 0039+4302. The observations were obtained using the high-speed multichannel photometer ULTRACAM on the 4.2-m William Herschel Telescope with a sampling interval of 1 and 4 s, respectively, for the two targets. Both targets show multiperiodic oscillations, generally considered to be due to p-mode pulsations. The frequency spectra are similar to those observed previously, although the amplitudes of some modes have altered. The observations are compared with theoretical multicolour light curves for non-radially oscillating extreme horizontal branch stars using the amplitude ratio method. The radial and spherical degrees n and l of all unambiguously identified frequencies have been determined. In general, n less than or equal to 3 and l less than or equal to 2, but both stars show one l = 4 mode. The spectra of frequency versus spherical degree are compared with models for evolved extended horizontal branch stars. These confirm that KPD 2109+4401 has a post-zero-age horizontal branch age of approximately 47 Myr and an envelope mass similar to0.0002 M.. HS 0039+4302 lies on the upper edge of the horizontal branch and hence its envelope mass and age are less well determined spectroscopically. The pulsation properties suggest a more massive envelope and evolved structure. In both stars, the frequencies of the radial (l = 0) modes, if correctly identified, do not match the selected models well, suggesting that the density structure or opacity in the stellar envelopes may be incorrect.