The Optical Gravitational Lensing Experiment. Cepheids in the Magellanic Clouds. II. Single-Mode Second Overtone Cepheids in the Small Magellanic Cloud

We present a sample of 13 candidates for single-mode second overtone Cepheids discovered in the 2.4 square degree area of the SMC bar during the OGLE microlensing search. All candidates passed four photometric tests: analysis of Fourier parameters of the light curve, R_21 and \phi_21, position in the period-luminosity diagram and location in the color-magnitude diagram. The stars form a very homogeneous group of objects with very similar light curves of low amplitude and almost sinusoidal shape. Their color indices B-V and V-I are also very similar and the objects determine the observational blue edge of the instability strip of the first overtone Cepheids. This is the first firm sample of the single-mode second overtone Cepheids detected so far.