EROS VARIABLE-STARS - FUNDAMENTAL-MODE AND FIRST-OVERTONE CEPHEIDS INTHE BAR OF THE LARGE-MAGELLANIC-CLOUD

We present CCD phase-binned light curves at 490 nm for 97 Cepheid vari able stars in the bar of the LMC. The photometry was obtained as part of the French EROS project and has excellent phase coverage, permittin g accurate decomposition into Fourier components. We identify as 'sinu soidal' or s-Cepheids those stars with periods less than 5.5 d and sma ll second-harmonic components. These stars comprise similar to 30% of our sample and most form a sequence similar to 1 mag brighter than the LMC classical Cepheids in the period-luminosity diagram. They are als o generally bluer and have lower-amplitude light curves. We infer that the s-Cepheids are first-overtone pulsators because, when their perio ds are converted to expected fundamental-mode values, they obey a comm on period-luminosity-colour relation with classical Cepheids. This als o confirms the reality of the colour term in the Cepheid period-lumino sity-colour relation. Further, the blue edge of the classical Cepheid instability strip agrees well with the theoretical calculations for th e fundamental mode made by Chiosi et al. (1993) for the Hertzsprung-Ru ssell and period-luminosity diagrams, but we find that our observed s- Cepheids are > 0.2 mag brighter and bluer than the Chiosi et al. predi ctions for the first-overtone. We identify a number of features in plo ts of our stars' Fourier-component amplitude ratios and phase differen ces. These features have been identified with resonances between diffe rent pulsation modes. In the LMC we find these features seem to occur at periods very similar to Galactic ones for classical Cepheids, but a t different periods for s-Cepheids. We discover a double-mode Cepheid in the LMC, for which P(first overtone)/P(fundamental) = 0.710+/-0.001 , very similar to observed ratios for Galactic double-mode Cepheids.