Nifty hovers around 8,350 ahead of May F&O expiry: Markets have started the session on a flat note with positive bias aided by global cues. Caution prevails on the bourses as investors remain wary ahead of today’s May Futures & Options (F&O) expiry. At 9:30 am, the Sensex was at 27,606, higher by 42 points or 0.2% and the Nifty was at 8,346, up 11 points. The Indian rupee has opened at Rs 63.93 a dollar today, up 8 paise over previous day's close. In the previous session, the currency closed at 64.01 a dollar, the lowest closing level since May 12. The dollar retained its strength sparked after remarks from Federal Reserve Chair Janet Yellen last Friday that she expected the central bank to raise rates this year as the US economy was on course to bounce back from a sluggish first quarter.