Kepler observations of the pulsating subdwarf B star KIC 2697388: the detection of converging frequency multiplets in the full data set

The Kepler spacecraft observed ∼150 000 stars over the course of its four-year mission, of which 18 were discovered to be pulsating subdwarf B stars, including KIC 2697388. We analyse three years of Kepler spacecraft short-cadence data as well as 21 low-resolution spectra of the pulsating subdwarf B star KIC 2697388. Our spectra have a radial-velocity scatter of 9.5 km s−1, and while insufficient to completely rule out binarity, we rule out short-period, low-inclination orbits for KIC 2697388. From the short-cadence Kepler data, we detect 253 periodicities, most with periods from 1 to 2.5 h, which we associate with gravity-mode pulsations. Twenty-three periods were also detected in the short-period pressure-mode region. We applied standard seismic tools for mode identification, including asymptotic overtone period spacings and rotationally induced frequency multiplets. We classify 89 per cent of the periodicities with mode identifications; most of low degree (ℓ ≤ 2), but 42 are identified as ℓ ≥ 3. Frequency multiplets provide a rotation period for the star of ∼42 d. A unique feature is seen in KIC 2697388\u27s data; in all ℓ ≥ 2 multiplets, the splittings decrease over the course of the observations. If the trend continues, ℓ ≥ 2 multiplets would become singlets within a decade