Electromagnetic effects on the light pseudoscalar mesons and determination of $m_u/m_d$

The MILC Collaboration has completed production running of electromagnetic effects on light mesons using asqtad improved staggered quarks. In these calculations, we use quenched photons in the noncompact formalism. We study four lattice spacings from $\approx\!0.12\:$fm to $\approx\!0.045\:$fm. To study finite-volume effects, we used six spatial lattice sizes $L/a=12$, 16, 20, 28, 40, and 48, at $a\!\approx\!0.12\:$fm. We update our preliminary values for the correction to Dashen's theorem ($ε$) and the quark-mass ratio $m_u/m_d$.