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Context. The discovery of pulsations in low-mass stars opens an opportunityfor probing their interiors and to determine their evolution, by employing thetools of asteroseismology. Aims. We aim to analyze high-speed photometry ofSDSSJ145847.02$+$070754.46 and SDSSJ173001.94$+$070600.25 and discoverbrightness variabilities. In order to locate these stars in the $T_{\rm eff} -\log g$ diagram we fit optical spectra (SDSS) with synthetic non-magneticspectra derived from model atmospheres. Methods. To carry out this study, weused the photometric data obtained by us for these stars with the 2.15mtelescope at CASLEO, Argentina. We analyzed their light curves and we apply theDiscrete Fourier Transform to determine the pulsation frequencies. Finally, wecompare both stars in the $T_{\rm eff} - \log g$ diagram, with known twopre-white dwarfs, seven pulsating pre-ELM white dwarf stars, $\delta$ Scuti andSX Phe stars. Results. We report the discovery of pulsations inSDSSJ145847.02$+$070754.46 and SDSSJ173001.94$+$070600.25. We determine theireffective temperature and surface gravity to be $T_{\rm eff}$ = 7 972 $\pm$ 200K, $\log g$ = 4.25 $\pm$ 0.5 and $T_{\rm eff}$ = 7 925 $\pm$ 200 K, $\log g$ =4.25 $\pm$ 0.5, respectively. With these parameters these new pulsatinglow-mass stars can be identified with either ELM white dwarfs (with ~ 0.17 Mo)or more massive SX Phe stars. We identified pulsation periods of 3 278.7 and 1633.9 s for SDSSJ145847.02$+$070754.46 and a pulsation period of 3 367.1 s forSDSSJ173001.94$+$070600.25. These two new objects together with those of Maxtedet al. (2013, 2014) indicate the possible existence of a new instability domaintowards the late stages of evolution of low-mass white dwarf stars, althoughtheir identification with SX Phe stars cannot be discarded.

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