What is known about the pair of planets orbiting Gliese 581 that Stéphane Udry and his colleagues found, which they believe could harbor life?
I believe you are talking about Gliese 581 b and Gliese 581 c.
Gliese 581 b, the first discovered (2005), has been overshadowed by Gliese (Gl) 581 c (2007). Gl 581 b is a Neptune sized planet with a solar orbit in Gliese 581's habitable zone: Gliese 581 is the red dwarf star that the planets orbit around. Gl 581 b has an unmeasured radius but an orbit period of 5.36865 days. Its size and orbital proximity to sun Gliese 581 reduces its likelihood for consideration as a habitable planet.
Gl 581 c was discovered in 2007 and is a "Super Earth" (larger than Earth but not as large as Neptune) within Gl 581's habitable zone. At first thought a prime candidate for habitability, the thought was later thrown into question because of potentially unsafe atmospheric gases due to runaway greenhouse effect wherein oceans evaporate at an accelerated rated. Originally, Gl 581 c was not directly detected but rather discovered through indirect observation related to sun Gl 581's gravitational wobble caused by gravitational pull from its planets. No radius is calculated. Its minimal mass is 5.073 and it is approximately 29 Earth masses. Its mass was calculated using the minimum mass of directly detected Gl 581 b and the assumed, but not directly detected, Gl 581 d.
In the meantime, scientists have further discovered Gl 581 e, Gl 581 f, and Gl 581 g. It is Gl 581 g that is currently the prime candidate for habitability. Gl 581 g was discovered by Dr. Ragbir Bhatha and was announced in 2011. Bhatha discovered Gl 581 g because of a "suspicious" area in the Gl 581 solar system. After the discovery of Gl 581 g, Gl 581 e was also discovered. At this time, Gl 581 g is thought to be the most likely Gl 581 planet for habitability though it has a 37 day solar orbit and no planetary rotation, therefore having no days and nights: it has a perpetual dark side and an everlasting day side, just like Earth's moon.