Compounds of formula (I) and their pharmaceutically acceptable salts are usefuf as histamine H3 receptor ligands. R?1 and R3¿ are optional substituents such as C¿1? to C6 alkyl. R?2¿ represents a bond or C¿1? to C5 (preferably C1 to C3) hydrocarbylene; R?4¿ represents a bond or C¿1? to C5 (preferably C1 to C3) hydrocarbylene; R?5¿ is hydrogen, C¿1? to C3 alkyl, R?8-O-(C¿1 to C3)alkyl (wherein R8 is hydrogen or C¿1? to C3 alkyl), aryl, aryl(C1 to C3)alkyl or heterocyclic (C1 to C3)alkyl; R?6¿ represents a bond or -NR9-, wherein R9 is any of the groups mentionned above for R5; R7 is H or C¿1? to C15 hydrocarbyl (in which one or more hydrogen atoms may be replaced by halogen, and up to three carbon atoms may be replaced by oxygen, nitrogen or sulfur atoms); a is from 0 to 2; and b is from 0 to 3.