  We consider bound states in the continuum (BICs) in low-contrast dielectric
gratings (DGs). It is demonstrated that the BICs originate from the reduced
guided modes on the effective dielectric slab with the permittivity equal to
the average permittivity of the DG. In case of isolated resonances the
positions of BICs can be found from two-wave dispersion relationships for
guided leaky modes. In the case of the degeneracy between the two families of
leaky modes the system exhibits an avoided crossing of resonances. In the
spectral vicinity of the avoided crossing the transmittance as well as the
emergence of BICs is described in the framework of the generic formalism by
Volya and Zelevinsky [Physical Review C 67, 054322 (2003)] with a single
fitting parameter.
