  Solar and stellar flares are powerful events which produce intense radiation
across the electromagnetic spectrum. Multiwavelength observations are highly
important for understanding the nature of flares, because different
flare-related processes reveal themselves in different spectral ranges. To
study the correlation between thermal and nonthermal processes in stellar
flares, we have searched the databases of Kepler (optical observations) and
XMM-Newton (soft X-rays) for the flares observed simultaneously with both
instruments; nine distinctive flares (with energies exceeding $10^{33}$ erg) on
three stars (of K-M spectral classes) have been found. We have analyzed and
compared the flare parameters in the optical and X-ray spectral ranges; we have
also compared the obtained results with similar observations of solar flares.
Most of the studied stellar flares released more energy in the optical range
than in X-rays. In one flare, X-ray emission strongly dominated, which could be
caused either by soft spectrum of energetic electrons or by a near-limb
position of this flare. The X-ray flares were typically delayed with respect to
and shorter than their optical counterparts, which is partially consistent with
the Neupert effect. Using the scaling laws based on the magnetic reconnection
theory, we have estimated the characteristic magnetic field strengths in the
stellar active regions and the sizes of these active regions as about $25-70$ G
and $250\,000-500\,000$ km, respectively. The observed stellar superflares
appear to be scaled-up versions of solar flares, with a similar underlying
mechanism and nearly the same characteristic magnetic field values, but with
much larger active region sizes.
