The transient response of Ba(Fe1−xCox)2As2, x=0.08 was studied by pump-probe optical reflectivity. After ultrafast photoexcitation, hot electrons were found to relax with two different characteristic times, indicating the presence of two distinct decay channels: a faster one, of less than 1 ps in the considered pump fluence range, and a slower one, corresponding to lattice thermalization and lasting ∼ = 6ps. Our analysis indicates that the fast relaxation should be attributed to preferential scattering of the electrons with only a subset of the lattice vibration modes, with a second moment of the Eliashberg function λ ω 2 ∼ = 64 meV 2 . The simultaneous excitation of a strong fully symmetric A1g optical phonon corroborates this conclusion and makes it possible to deduce the value of λ ∼ = 0.12. This small value for the electron-phonon coupling confirms that a phonon mediated process cannot be the only mechanism leading to the formation of superconducting pairs in this family of pnictides. PACS numbers: 74.70.Xa; 78.47.J-; 74.25.KcThe discovery of high temperature superconductivity in iron pnictide compounds in 2008(Refs. 1 and 2 ) has raised a lot of questions about the nature of this phenomenon. One of these questions concerns the role of electron-phonon (e-ph) coupling, which is at the heart of conventional Bardeen-Cooper-Shrieffer (BCS) superconductivity 3 .For unconventional superconductors, electron-lattice interaction mechanisms have been extensively studied in cuprates; these materials present some similarities with pnictides, such as the bidimensionality of the crystallographic structure with Cu-O planes instead of Fe-As ones and the presence of a magnetic phase in the underdoped part of the phase diagram 4,5 . While the electron pairing mechanism is still controversial in superconducting cuprates, some elements are today accepted: first, their high critical temperatures are not compatible with a BCS scheme; second, the e-ph interaction is anisotropic, as theoretically predicted 6 and verified by means of time-resolved experiments 7-11 ; indeed, the e-ph coupling constant is strongly mode-selective, and ranges from λ ∼ = 0.13 to λ ∼ = 0.55. This selectivity is linked to the marked bidimensional layered structure of cuprates, leading to a preferential coupling between electrons coming from a specific k-direction of the Fermi surface and one particular phonon mode 6,8 .Much less information is available on e-ph coupling in pnictides. Theoretical works on the 1111 family (LaFeAsO 1−x F x ), employing Density Functionnal perturbation theory, predicted an isotropic coupling, equally distributed for the whole phonon population, and too small to be responsible for superconductivity through a BCS type mechanism 5,12 . On the other hand, a very strong coupling (λ ∼ = 1) between electrons and the A 1g mode (consisting of a breathing movement of As atoms) was predicted by a model in which the electronic polarization of As atoms involve e-ph interaction 13 . For 122 compounds (doped AF e 2 As 2 , A=Ba, Sr, Ca) spin f...