Multicolour high-speed photometry of the subdwarf B star PG 0014+067 with ULTRACAM

The non-radially pulsating subdwarf B star PG 0014+067 has previously been presented as a classic case for asteroseismological study, having a moderately rich mode spectrum uncomplicated by severe rotational splitting. Notwithstanding the excellence of previous work, empirical evidence for mode identification is needed to test the modelling results. Consequently high-speed multicolour photometry was obtained over six nights in 2004 August using the high-speed multichannel photometer ULTRACAM on the 4.2-m William Herschel Telescope with a sampling interval of 5 s. To ameliorate the window function, additional single-channel photometry was obtained on five nights in the same time envelope using the 2-m Faulkes Telescope North with a sampling interval of 23 s. 19 individual frequencies have been identified in a combined ‘white light’ data set. Amplitudes have been measured in filtered light (u′, g′ and r′) for 13 of these. Three groups of closely spaced frequencies have spacings of 3, 13 and 2 μHz. Although on the cusp of the frequency resolution, there is evidence that the rotational period should be nearer to 4 d rather than the 1.35 d reported previously, if we assume that these close frequencies belong to a multiplet. It has not proved possible to identify modes unambiguously using the amplitude ratio method because of the errors on the amplitudes, but we do exclude that the two dominant modes have l= 3 or 4; they must be either l= 0, 1, 2.