especially you have signed the offer letter.
The rule is (1). For professor, if he/she issued you an offer letter, he/she should not change idea even he/she found stronger candidant; (2) FOR YOU, the same rule, once yiou signed the letter, you should keep your promise.
However, if you have a reasonable explaination, that's another story
The rule is (1). For professor, if he/she issued you an offer letter, he/she should not change idea even he/she found stronger candidant; (2) FOR YOU, the same rule, once yiou signed the letter, you should keep your promise.
However, if you have a reasonable explaination, that's another story