= Solution
The stationary <Killing vector field> is $k=\partial_t=\partial_v$, with $k^2=-f$. The normal to $r=\text{constant}$ has squared norm $g^{rr}=f$, so $r=r_+$ is a <null hypersurface>; there $k$ is both tangent and normal. It is therefore a <Killing horizon>.
For a static metric of this form, the <surface gravity> is $\kappa=f'(r_+)/2$. Since $r_+^2r_-^2=Q^2$ and $r_+^2+r_-^2=2M$,
$$
\boxed{\kappa=\frac{r_+^2-r_-^2}{r_+^3}=\frac{2\sqrt{M^2-Q^2}}{r_+^3}}.
$$
For the <extremal black hole> $M=Q$, the horizons coincide and \b[$\kappa=0$].
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