[SamanthaG]

Hi,I'm a lawyer (in the UK) and you do need a legally binding agreement in place to protect you. I'm sure someone at Y combinator can recommend a decent lawyer over there. You should not have to pay a great deal for this, but it is best to have it done professionally so that in the worse case scenario (that you guys fall out with each other), you're legally sorted from the outset. Its probably not a good idea to try and do this yourself. I do mean that sincerely and not in a patronising way at all, you want to be able to focus on your start-up and not be worrying about what would happen if things went wrong. Good luck.