2008A&A...480..187D

The λ Bootis stars are a chemically peculiar subgroup of main sequence A/F stars. Several of these fall into the instability strip of the δ Scuti stars and the whole spectroscopic group shows a high incidence of pulsational variability. We investigated the pulsational behaviour of the λ Bootis star 15 And (HD 221756) to determine as many intrinsic mode frequencies as possible for asteroseismic applications. The star was included as a secondary target for three telescopes during a campaign devoted to the β Cep stars 12 and 16 Lac, due to its proximity to these targets. 302.8h of time-resolved photometry spanning 161.2 d were acquired and analyzed. Our frequency analysis revealed f₁=24.7887(2)c/d as the main pulsation frequency, with an amplitude of only 1.9(1)mmag. The residual periodogram suggests the presence of additional frequencies in an interval from 20 to 25c/d, with very low amplitudes. We succeeded in detecting a second frequency f₂=22.2580(9) with an amplitude of 0.6(1)mmag. Low-frequency light variations of the comparison star HD 220575 were revealed as well. The pulsational behaviour of 15 And as a typical λ Boo-type star is compared with that of δ Scuti stars with normal abundances and with metallic-line δ Scuti stars. We discuss the value of λ Boo stars in understanding accretion and mixing in pulsating stars. The difficulties in the selection of reliable comparison stars for such high-precision research are pointed out.