If I enter the mosque and find the Imam standing in the second rak'ah, do I only say the opening takbeer (takbeerat al-ihram), or do I say the opening takbeer and the takbeer of transition from prostration to standing?

If a latecomer finds the Imam standing, he should only pronounce the opening Takbir (Takbirat al-Ihram) and no other Takbir for transitioning, as there is no place for it here. Similarly, if he catches the Imam prostrating or in Tashahhud, he should pronounce the Takbirat al-Ihram while standing, then sit or prostrate without pronouncing a Takbir for transitioning, according to the sounder opinion.