  Pulsating extremely low-mass pre-white dwarf stars (pre-ELMV), with masses
between ~0.15 Msun and ~0.30 Msun, constitute a new class of variable stars
showing 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 delta
Scuti stars nearly overlap in the Teff vs. log g diagram, leading to a
degeneracy when spectroscopy is the only tool to classify the stars and
pulsation periods only are considered. We employ adiabatic and non-adiabatic
pulsation for models of pre-ELM and delta Scuti stars, and compare their
pulsation periods, period spacings and rates of period change. We found
substantial differences in the periods spacing of delta Scuti and pre-ELM white
dwarf models. Even when the same period range is observed, the modes have
distinctive signature in the period spacing and period difference values. For
instance, the mean period difference of p- modes of consecutive radial orders
for delta Scuti model are at least four times longer than the mean period
spacing 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 for
the pre-ELM white dwarfs compared to delta Scuti stars. In addition, we also
report the discovery of a new variable star, SDSSJ075738.94+144827.50, located
in the region of the Teff vs. log g diagram where these two kind of stars
coexist. The characteristic spacing between modes of consecutive radial orders
(p as well as g modes) and the large differences found in the rates of period
change for delta Scuti and pre-ELM white dwarf stars suggest that
asteroseismology can be employed to discriminate between these two groups of
variable stars.
