The centres of two circles $${C_1}$$ and $${C_2}$$ each of unit radius are at a distance of 6 units from each other. Let P be the mid point of the line segement joining the centres of $${C_1}$$ and $${C_2}$$ and C a circle touching circles $${C_1}$$ and $${C_2}$$ externally. If a common tangent to $${C_1}$$ and passing through P is also a common tangent to $${C_2}$$ and C, then the radius of the circle C is