BERLIN, Oct 8 (Reuters) - German real estate firm Vonovia said on Friday it had bought an option to acquire around 13% stake in smaller rival Adler as its shares fell as much as 33% on Wednesday after an activist investor said it was “not investible”. Vonovia said the option gives it the right to buy Adler shares from Luxembourg-based property investor Aggregate Holdings over the next 18 months at a strike price of 14 euros ($16.16) per share. ($1 = 0.8661 euros) (Reporting by Zuzanna Szymanska Editing by Riham Alkousaa)