  Using a geometric approach involving Riemann surface orbifolds, we provide
lower bounds for the genus of an irreducible algebraic curve of the form
$E_{A,B}:\, A(x)-B(y)=0$, where $A, B\in\mathbb C(z)$. We also investigate
"series" of curves $E_{A,B}$ of genus zero, where by a series we mean a family
with the "same" $A$. We show that for a given rational function $A$ a sequence
of rational functions $B_i$, such that ${\rm deg}\, B_i \rightarrow \infty$ and
all the curves $A(x)-B_i(y)=0$ are irreducible and have genus zero, exists if
and only if the Galois closure of the field extension $\mathbb C(z)/\mathbb
C(A)$ has genus zero or one.
