In such cases (called corporate actions), the stock exchange will make adjustment to F&O contracts. These changes come in to force from the day these actions are implemented i.e. next trading day after record date. Date of news announcement has no effect on it. Though based on the news, people start finding arbitrage opportunity and encash on it. But it settles down over a period of time.
In case on 1:1 bonus, number of share will double in the market and hence trading price gets halved. So the Lot size of contract will also double and trading price of option will come down to half. Take an example
2700 call with lot size of 200 ( 2700*200 = 5.4Lac worth) traded @ 50 rs. * 200 = 10k.
i.e with 10k you are controlling 5.4 lac of infy stocks.
After bonus implementation
1350 call with lot size of 400 (1350 * 400= 5.4lac) traded @ X rs * 400 = 10k.
which gives u X = 25 rs price of option. As the lot size has doubled, your investment of 10k remains unaffected.
Till the bonus record date, it is business as usual with not change in trading. Your positions will gain/loss as they were anyway doing till yesterday or before bonus announcement.
Hope this helps.
You can refer to following link to NSE site that explains in detail about the adjustment in F&O contracts.
http://www.nse-india.com/content/nsccl/nsccl_focorpactions.htm
Happy Trading