  We prove local boundedness and H\"older continuity for weak solutions to
nonlocal double phase problems concerning the following fractional energy
functional \[ \int_{\mathbb{R}^n}\int_{\mathbb{R}^n}
\frac{|v(x)-v(y)|^p}{|x-y|^{n+sp}} + a(x,y)\frac{|v(x)-v(y)|^q}{|x-y|^{n+tq}}\,
dxdy, \] where $0<s\le t<1<p \leq q<\infty$ and $a(\cdot,\cdot) \geq 0$. For
such regularity results, we identify sharp assumptions on the modulating
coefficient $a(\cdot,\cdot)$ and the powers $s,t,p,q$ which are analogous to
those for local double phase problems.
