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Pulsating extremely low-mass pre-white dwarf stars (pre-ELMV), with massesbetween ~0.15 Msun and ~0.30 Msun, constitute a new class of variable starsshowing g- and possibly p-mode pulsations with periods between 320 and 6000 s,while main sequence delta Scuti stars, with masses between 1.2-2.5 Msun,pulsate in low-order g and p modes with periods in the range [700-28800] s.Interestingly enough, the instability strips of pre-ELM white dwarf and deltaScuti stars nearly overlap in the Teff vs. log g diagram, leading to adegeneracy when spectroscopy is the only tool to classify the stars andpulsation periods only are considered. We employ adiabatic and non-adiabaticpulsation for models of pre-ELM and delta Scuti stars, and compare theirpulsation periods, period spacings and rates of period change. We foundsubstantial differences in the periods spacing of delta Scuti and pre-ELM whitedwarf models. Even when the same period range is observed, the modes havedistinctive signature in the period spacing and period difference values. Forinstance, the mean period difference of p- modes of consecutive radial ordersfor delta Scuti model are at least four times longer than the mean periodspacing for the pre-ELM white dwarf model in the period range [2000 - 4600] s.In addition, the rate of period change is two orders of magnitudes larger forthe pre-ELM white dwarfs compared to delta Scuti stars. In addition, we alsoreport the discovery of a new variable star, SDSSJ075738.94+144827.50, locatedin the region of the Teff vs. log g diagram where these two kind of starscoexist. The characteristic spacing between modes of consecutive radial orders(p as well as g modes) and the large differences found in the rates of periodchange for delta Scuti and pre-ELM white dwarf stars suggest thatasteroseismology can be employed to discriminate between these two groups ofvariable stars.

Comparing the asteroseismic properties of pulsating pre-extremely low mass white dwarf and delta Scuti stars