Coffee and tea are allowed during the fast. I've been drinking a lot more tea recently (I'm not a huge coffee drinker). There's a certain threshold where your body kicks into "metabolic" mode. The exact figure varies, but I think you want to avoid eating > ~50 calories[1] or your digestion kicks in and your fast is ruined. A cup of tea or coffee, for example, is around 3 calories. Even milk, in small quantities, is probably fine. It goes without saying that sugar is not allowed.

[1] My figures are probably wrong. Someone feel free to correct me.