  It is proposed that sources of repeating fast radio bursts with detected
periodicity are magnetars with extremely short initial spin periods at the
protoneutron star stage, formed in binaries after tidal synchronization of
their progenitor cores on late stages of thermonuclear burning in very close
systems with orbital periods about a few days. This formation channel is in
several respects different from evolution of progenitors of normal magnetars,
and thus, it explains some differences between repeating and non-repeating
sources of FRBs.
