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Astron. Astrophys. 332, L53-L56 (1998)

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1. Introduction

The period-luminosity (PL) relation for LMC Miras was discovered by Glass & Lloyd Evans (1981) and subsequently refined by Feast (1984) and Feast et al. (1989). According to these authors, the relation is tight with a dispersion of [FORMULA] It is shown in Fig. 1b as the continuous line deduced from LMC Miras observations. A slight revision for carbon Miras was published by Groenewegen & Whitelock (1996). Their zero point of the PL relation relied on the galactic binary member UV Aur. van Leeuwen et al. (1997) made use of the HIPPARCOS data on 16 galactic Miras, including one carbon star (R Lep). They obtained [FORMULA] for the LMC distance modulus which is close to the 18.5 value frequently adopted from other methods. Our analysis of carbon-rich variables gives [FORMULA] (Sect. 4). This result is obtained making use of a sample of 72 galactic carbon LPVs with known periods (Sample 1), out of 115 stars observed in 1989-1993 by HIPPARCOS. They are identified in Fig. 1 as the analogs of the LMC variables concentrated in a clump close to the PL relation of Feast et al. (1989). An upper limit (ul) is also observed which is shown at long periods in both diagrams. Short period stars located markedly above the PL relation (Sample 2: possible overtone pulsators according to the analysis of Wood & Sebo 1996 who identified the LMC clump stars as fundamental pulsators). Underluminous LPVs (Sample 3) are observed in both galaxies. Expectations of true parallaxes generated by Knapik et al. (1997) are used since, contrary to observed parallaxes, they are free of bias induced by errors and non-uniform distribution (which is part of the LK effect). No truncation was operated. It was also shown that the Malmquist bias should be negligible for a sample of bright HIPPARCOS LPVs [FORMULA] provided the estimated true parallaxes are larger than 0.6 mas which is the case here. Making use of the [FORMULA] in the parallax space (Sect. 4) yields the same sampling as the non-gaussian absolute magnitudes. The slope and intercept of the PL relation are simultaneously derived making use of a non-linear parametric method. Finally, the LMC distance modulus is deduced and compared to published values.

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© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998