Marquette, J. B.Tisserand, PatrickFrançois, P.Beaulieu, J.-P.Doublier, V.Lesquoy, E.Milsztajn, A.Pritchard, J.Schwarzenberg-Czerny, A.Afonso, C.Albert, J. N.2015-12-080004-6361http://hdl.handle.net/1885/33251Context. For seven years, the EROS-2 project obtained a mass of photometric data on variable stars. We present a peculiar Cepheid-like star, in the direction of the Small Magellanic Cloud, which demonstrates unusual photometric behaviour over a short time interval. Aims. We report on data of the photometry acquired by the MARLY telescope and spectroscopy from the EFOSC instrument for this star, called EROS2 J005135-714459(sm.0060n13842), which resembles the unusual Cepheid HR 7308. Methods. The light curve of our target is analysed using the Analysis of Variance method to determine a pulsational period of 5.5675 days. A fit of time-dependent Fourier coefficients is performed and a search for proper motion is conducted. Results. The light curve exhibits a previously unobserved and spectacular change in both, mean magnitude and amplitude, which has no clear theoretical explanation. Our analysis of the spectrum, implies a radial velocity of 104 km s-1 and a metallicity of -0.4 ± 0.2 dex. In the direction of right ascension, we measure a proper motion of 17.4 ± 6.0 mas yr-1 using EROS astrometry, which is compatible with data from the NOMAD catalogue. Conclusions. The nature of EROS2 J005135-714459(sm0060n13842) remains unclear. For this star, we may have detected a non-zero proper motion, which would imply that it is a foreground object. Its radial velocity, pulsational characteristics, and photometric data, however, suggest that it is instead a Cepheid-like object located in the SMC. In such a case, it would present a challenge to conventional Cepheid models.Keywords: Astronomy; Electric currents; Imaging techniques; Photography; Photometry; Regression analysis; Analysis of variance (ANOVA) method; Cepheid; Fourier coefficients; Light curves; Photometric data; Proper motions; Radial velocity (RV); Short time; Small mag Cepheids; Galaxy: disk Magellanic clouds stars: Peculiar200810.1051/0004-6361:200669152015-12-08