  Regular intrinsic brightness variations observed in many stars are caused by
pulsations. These pulsations provide information on the global and structural
parameters of the star. The pulsation periods range from seconds to years,
depending on the compactness of the star and properties of the matter that
forms its outer layers. Here, we report the discovery of more than a dozen of
previously unknown short-period variable stars: blue large-amplitude pulsators.
These objects show very regular brightness variations with periods in the range
of 20-40 min and amplitudes of 0.2-0.4 mag in the optical passbands. The phased
light curves have a characteristic sawtooth shape, similar to the shape of
classical Cepheids and RR Lyrae-type stars pulsating in the fundamental mode.
The objects are significantly bluer than main sequence stars observed in the
same fields, which indicates that all of them are hot stars. Follow-up
spectroscopy confirms a high surface temperature of about 30,000 K. Temperature
and colour changes over the cycle prove the pulsational nature of the
variables. However, large-amplitude pulsations at such short periods are not
observed in any known type of stars, including hot objects. Long-term
photometric observations show that the variable stars are very stable over
time. Derived rates of period change are of the order of 10^-7 per year and, in
most cases, they are positive. According to pulsation theory, such
large-amplitude oscillations may occur in evolved low-mass stars that have
inflated helium-enriched envelopes. The evolutionary path that could lead to
such stellar configurations remains unknown.
