Mumbai, May 19 (IANS) The secret behind the chemistry between Vidya Balan and Madhavan in the Airtel ads is out. It is Madhavan's two-year-old son Vedant.

The child bonded so beautifully with Vidya on the sets of 'Guru' that she became a part of Madhavan and his wife Sarita's inner circle.

In fact the cell phone company has now decided to put out a series of ads featuring Madhavan and Vidya as a couple connecting emotionally on the phone. Madhavan has been running in and out of Mumbai to complete these ads.

'Initially I was only the brand ambassador for Airtel's southern market. But now they've signed me on for the whole country,' Madhavan told IANS.

'We've shot a series of ads with Vidya and me. For the last one week I've been running in and out Mumbai and Chennai. During the daytime I'm in Chennai shooting for a Tamil film. Then I take a night flight to Mumbai to dub for the ads,' the southern star said.

Curiously, Maddy, as Madhavan is fondly known, and Vidya have hardly shot together for these ads about love and togetherness.

'We've just done one shot together. The rest has been shot with the two of us separately. It's strange people have noticed so much chemistry between us when we aren't even together in the ad. My wife Sarita loves the ad. She thinks Vidya is better than me in the ad. But then she's my harshest critic. All three of us Sarita, Vedant and I adore Vidya.'

Vidya has done ads prior to her acting career. This is her first ad after starring in films.

She thoroughly enjoyed the experience. 'Years ago I was supposed to do a Tamil film with Maddy. Then we were also supposed to do a Deepa Mehta film together. I think Maddy and I were destined to special things in limited space,' says Vidya.

'Both of us had cameos in Mani Ratnam's 'Guru'. People still talk about our romantic scene in the rain (including a kiss). And now the Airtel ads. It's unbelievable how much chemistry people have seen between us in those few seconds. I always say it's the moments that make a romance.'