  Gamma-ray bursts (GRBs) are among the most luminous sources in the universe
and the nature of their emission up to very high energy is one of the most
important open issue connected with the study of these peculiar events. The
High Altitude Water Cherenkov (HAWC) gamma-ray observatory, installed at an
altitude of 4100 m a. s. l. in the state of Puebla (Mexico), has completed its
second year of full operations. Thanks to its instantaneous field of view of ~2
sr and its high duty cycle ($\ge$ 95%), HAWC is an ideal instrument for the
study of transient phenomena such as GRBs. We performed a search for TeV
emission delayed with respect to, and of longer duration than the prompt
emission observed by satellites. We present here the results obtained by
observing at the position of a sample of GRBs detected by the Fermi and Swift
satellites from December 2014 to February 2017. The upper limits resulting from
this analysis are presented and theoretical implications are discussed.
