Two new pulsating low-mass pre-white dwarfs or SX Phoenicis stars?

Context. The discovery of pulsations in low-mass stars opens an opportunity to probe their interiors and determine their evolution by employing the tools of asteroseismology. Aims. We aim to analyse high-speed photometry of SDSS J145847.02+070754.46 and SDSS J173001.94+070600.25 and discover brightness variabilities. In order to locate these stars in the T-eff - log g diagram, we fit optical spectra (SDSS) with synthetic nonmagnetic spectra derived from model atmospheres. Methods. To carry out this study, we used the photometric data we obtained for these stars with the 2.15 m telescope at CASLEO, Argentina. We analysed their light curves and applied the discrete Fourier transform (FT) to determine the pulsation frequencies. Finally, we compare both stars in the Teff log g diagram, with two known pre-white dwarfs and seven pulsating pre-ELM white dwarf stars, delta Scuti, and SX Phe stars Results. We report the discovery of pulsations in SDSS J145847.02+070754.46 and SDSS J173001.94+070600.25. We determine their effective temperature and surface gravity to be Teff = 7972 +/- 200 K, log g = 4 : 25 +/- 0 : 5 and Teff = 7925 +/- 200 K, log g = 4 : 25 +/- 0 : 5, respectively. With these parameters, these new pulsating low-mass stars can be identified with either ELM white dwarfs (with +/- 0 : 17 M-circle dot) or more massive SX Phe stars. We identified pulsation periods of 3278.7 and 1633.9 s for SDSS J145847.02+070754.46 and a pulsation period of 3367.1 s for SDSS J173001.94+070600.25. These two new objects, together with those of Maxted et al. (2013, 2014), indicate the possible existence of a new instability domain towards the late stages of evolution of low-mass white dwarf stars, although their identification with SX Phe stars cannot be discarded.