  We present results of a BVI variability survey in the young open cluster NGC
457 based on observations obtained during three separate runs spanning almost
20 years. In total, we found 79 variable stars, of which 66 are new. The BVI
photometry was transformed to the standard system and used to derive cluster
parameters by means of isochrone fitting. Using the complementary H-alpha
photometry carried out in two seasons separated by over 10 years, we find that
the cluster is very rich in Be stars. In total, 15 stars in the observed field
of which 14 are cluster members showed H-alpha in emission either during our
observations or in the past. Most of the Be stars vary in brightness on
different time scales including short-period variability related most likely to
g-mode pulsations. A single-epoch spectrum of NGC457-6 shows that this Be star
is presently in the shell phase. The inventory of variable stars in the
observed field consists of a single BCep-type star, NGC457-8, 13 Be stars, 21
slowly pulsating B stars, seven DSct stars, one GDor star, 16 unclassified
periodic stars, 8 eclipsing systems and a dozen of stars with irregular
variability, of which six are also B-type stars. As many as 45 variable stars
are of spectral type B which is the largest number in all open clusters
presented in this series of papers. The most interesting is the discovery of a
large group of slowly pulsating B stars which occupy the cluster main sequence
in the range between V=11 and 14.5 mag, corresponding to spectral types B3 to
B8. They all have very low amplitudes and about half show pulsations with
frequencies higher than 3 c/d. We argue that these are most likely
fast-rotating slowly pulsating B stars, observed also in other open clusters.
