Does energy gravitate

Light rays are affected by gravity. 1919 Eddington experiment matched a prediction of general relativity.

 

Energy contributes to a thing's inertia, and to the amount is pulled by gravity, exactly in the proportion you would expect. energy = c² times inertia
inertia = energy/c²

So if you have an iron canonball and you heat it, so it absorbs a little energy. Then its weight will increase proportionally by that amount. (that is, how much it is pulled)
and its inertia will increase in the same proportion as its weight does