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Astron. Astrophys. 354, L13-L16 (2000)

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1. Introduction

The hot subdwarf B (sdB) stars are low-mass stars ([FORMULA]) located near the extreme horizontal branch (Heber et al., 1984; Heber, 1986). Their helium (He) core, feeding the 3[FORMULA]-cycle nuclear fusion, is surrounded by a thin hydrogen (H) envelope ([FORMULA] [FORMULA]). Following spectroscopic and photometric studies several classification schemes can be found for the hot sds in general. According to Moehler et al. (1990b), the sdBs are He-poor stars which display optical spectra dominated by strong broad Balmer lines and weak, or absent, HeI absorption. A compilation of about 1200 hot sds can be found in Kilkenny et al. (1988). The hot subdwarfs represent in an emphasized way the classical horizontal branch problem, namely: how can a red giant of a given mass lose a substantial fraction of its H-rich envelope at or soon after the He ignition in its core? (Jeffery and Pollacco 1998). In the course of their evolution, a fraction of the hot sds is expected to form white dwarf (WD) stars with lower than average masses (Heber, 1986). Hence, a study of such stars is extremely important from an evolutionary point of view. The discovery that 13 sdB stars pulsate (O'Donoghue et al. 1999) has rapidly increased the interest of the astronomical community for these objects, since their interior can now be probed by seismological investigation. At about the same time of their discovery, investigations of the pulsational instability of the sdB stars were reported by Charpinet et al. (1996), the pulsation driving mechanism being due to an opacity bump associated with iron ionization. The observational properties of the sdB pulsating stars (called EC 14026 stars from the prototype EC 14026-2647, Kilkenny et al. 1997) are the following: periods between about 1 and 10 min, amplitudes between a few millimag to a few hundredths of mag. In addition at least 5 of them out of 13 are in binary systems (O'Donoghue et al. 1999). This percentage is not far from the 40-50[FORMULA] binarity rate for the hot subdwarf population (Ulla and Thejll 1998 and references therein); therefore it is unlikely, but not impossible, that binary companions play a direct role in the sdB pulsations. The oscillation properties of sdB models, now rapidly growing (Charpinet et al. 1997; Fontaine et al. 1998; O'Donoghue et al. 1998) predict that both radial and nonradial modes should have about same frequencies. It is possible, in principle, to distinguish between radial and nonradial modes: nonradial modes are affected by stellar rotation (they produce multiplets of almost equally spaced frequencies), whereas radial modes are not. The seismological investigation of the sdB stars could permit us to learn important details about their inner structure and chemical composition, as has been successfully done for several WD and pre-WD stars (Bradley 1998). Moreover, with long term measurements (months-years), we can hope to detect the variation of the pulsation period with time, which is directly related to the evolutionary changes of stellar structure.

Within the framework of our monitoring program of a hundred hot subdwarfs, in order to find eventual periodicities, in this letter we will present the results obtained for the sdB star PG 0856+121 ([FORMULA] = [FORMULA] [FORMULA] [FORMULA]; [FORMULA] = [FORMULA] [FORMULA] [FORMULA] by Colin et al. 1994). PG 0856+121 was classified as a typical sdB star by Moehler et al. (1990a) on the basis of its spectrum. The star does not show evidence of binarity, such as the IR CaII triplet and photometric red excess (Jeffery and Pollacco 1998). Moreover the [FORMULA] radial velocity is constant (Saffer et al. 1998) and the significance of the IR excess found for this object in the JHK bands by Ulla and Thejll (1998) does not reach a 2[FORMULA] level for all the three bands. The Strömgren photometry performed by Moehler et al. (1990a) gave [FORMULA], [FORMULA], [FORMULA], [FORMULA]. Wesemael et al. (1992) found photometric values in fairly good accordance with those obtained by Moehler et al. (1990a). Moehler et al. (1990b), from spectroscopy and photometry, derived [FORMULA], [FORMULA], [FORMULA] pc and [FORMULA] pc (distance from the galactic plane). Saffer et al. (1994) from optical spectrophotometry found fairly different values for temperature ([FORMULA]) and surface gravity ([FORMULA]). This fact renders evident the difficulty in determining the temperature of such stars using colour indices, independently of the photometric system used, since the central wavelength of the filters employed lies in the red-wing of the Planckian distribution, as also pointed out by Wesemael et al. (1992).

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© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000