5 Summary and discussion

The four new pulsating sdB stars presented here are all located well inside the region of the ( $T_{\rm eff}$, log g) plane where the sdB pulsators are expected, and have fairly simple temporal spectra, as far as we can judge from our short light curves.

HS0039+4302, with a $T_{\rm eff}$ of 32400 and log g of 5.7, is located right in the centre of the sdBV ( $T_{\rm eff}$, log g) distribution, comparable to PG1336-018 (Kilkenny et al. 1998). The main peak of PG1336-018 is found at 184s, with a secondary at 141s. For HS0039+4302 most of the power appears to be in a group of peaks between 195 and 182s, and a secondary group is seen at 242s. The rich pulsation spectrum and relatively strong amplitudes of this target makes it our favourite target for a multi-site campaign. HS0039+4302 is also the brightest among these new pulsators (B=15.1).

HS0444+0408, with $T_{\rm eff} \simeq 33$800K and log $g \simeq 5.6$, is similar to KPD1930+2752 (Billères et al. 2000). Campaign observations of KPD1930+2752 have revealed that its pulsation spectrum contains at least 44 periods located in a number of discrete groups between 145 and 332s (Billères et al. 2000). Our three short observing runs on HS0444+0408 reveal one clear pulsation around 137s, and at least one more close to 170s. Higher frequency resolution observations of this star could reveal if further periodicities are present.

HS1824+5745 has $T_{\rm eff} \simeq 33$100K and log $g \simeq 6.0$, which places it in the mid temperature - high gravity part of the sdBV ( $T_{\rm eff}$, log g) distribution, right next to EC10228-0905 (Stobie et al. 1997). Whereas we only detect a single peak at 139s in the amplitude spectrum of HS1824+5745, EC10228-0905 displays three peaks in the range 139-152s with the most significant peak at 139s. Also, the observations reported by Stobie et al. (1997) indicate that the amplitudes are variable, just as we can see for HS1824+5745. Thus, a comparison between these two stars indicates a link between ( $T_{\rm eff}$, log g) and pulsational behaviour.

HS2151+0857 has almost identical $T_{\rm eff}$, log g and pulsation periods as the class prototype EC14026-2647, which has at least two periods where one is stable and the other apparently variable in amplitude (Kilkenny et al. 1997). In the three nights we observed HS2151 the main peak remained stable, while at least one peak (at 7.72 mHz) varied from about 2.6 to 4.6 mma, giving at least some indication of corresponding amplitude variability to that of the prototype. The periods detected for HS2151+0857 (between 129 and 151s) also span nearly the same range as that of EC14026-2647 (134 and 144s).

The new pulsators all follow the log g-P relation (see Koen et al. 1999), as we see from Fig. 11, where log g has been plotted against the period of the pulsation with the strongest amplitude.

Figure 11: In this figure log g values are plotted versus the pulsation period with largest detected amplitude. The eight pulsators found during our NOT search-programme are represented with filled circles.

Acknowledgements

This research was supported by the Norwegian Research Council. R. S. acknowledges support by the italian research foundation MURST under project "Stellar Evolution'' (coordinator V. Castellani). H. E. acknowledges financial support by the german research foundation DFG under grant He 1354/30-1. The authors also wishes to thank the staff at the NOT for good support. S. D. and U. H. thank D. Engels, H. Hagen and D. Reimers (Hamburg) for a fruitful collaboration over many years.

The time-series data presented here have been taken using ALFOSC, which is owned by the Instituto de Astrofisica de Andalucia (IAA) and operated at the Nordic Optical Telescope under agreement between IAA and the NBIfAFG of the Astronomical Observatory of Copenhagen.

This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France.