Time for Krias HaMegillah

It is forbidden to read the Megillah at night  before the appearance of (three) stars,  even though one is greatly suffering due to the fast.  However, one may taste a little1 before the Megillah (reading),  such as coffee and similar (drinks),  in order to slightly strengthen oneself  from the weakness of the fast. (KSA 141:8)
1) One may eat the volume of a ''k'beitzah'' (lit: ''like an egg'') of bread and cake, or liquid in the same quantity. This is from 86.23 ml to 100 ml or in normal terms about two thirds of a slice of white bread or three quarters of a square matzah.