  We simulate 3-flavour lattice QCD at small isospin chemical potential $\mu_I$
and finite temperature $T$. At $\mu_I=0$ there is a critical mass $m_c$ where
the finite-temperature transition changes from first order to a crossover. We
measure the $\mu_I$ dependence of the transition $\beta$ ($\beta_c$) for $m$
close to $m_c$. $\beta_c$ and hence $T_c$ decrease slowly with increasing
$\mu_I$. $\beta_c$ at finite $\mu_I$ is in good agreement $\beta_c$ at finite
$\mu$ (quark-number chemical potential). We use fourth-order Binder cumulants
to determine the nature of this transition and to search for a critical
endpoint. We measure the $dt$ dependence of these cumulants and extrapolate to
$dt=0$. ($dt$ is the `time' increment used in the hybrid molecular-dynamics
simulations.) Preliminary measurements of these Binder cumulants show little
$\mu_I$ dependence. (Simulations at imaginary $\mu$ indicate that the $\mu$
dependence of the Binder cumulants is also weak.) This contrasts to the $\mu_I$
dependence we observed at fixed $dt$.
