t is 2;

each of X₁, X₃, and Y₁ represents CH or N and each of X₂, Y₂, and Y₃ represents CH, provided that X₁ and X₃ do not both represent N and provided that X₁ and Y₁ do not both represent N;

Ar¹ represents phenyl, said phenyl bearing 0 to 3 substituents selected from halogen, CN, CF₃, OCF₃, C_1-6alkyl, OH, C_1-6alkoxy or hydroxyC_1-6alkyl;

Ar² represents phenyl, pyridyl or thienyl, said phenyl, pyridyl or thienyl bearing 0 to 3 substituents selected from halogen, CN, nitro, R^a, OR^a, SR^a, SOR^a, SO₂R^a, SO₂NR^aR^b, (CH₂)_xNR^aR^b, (CH₂)_xNR^aCOR^b, (CH₂)_xNR^aCO₂R^b, (CH₂)_xNR^aCONR^aR^b (CH₂)_xNR^aSOR^b, (CH₂)_xNR^aSO₂R^b, (CH₂)_xNR^aSO₂NR^aR^b, (CH₂)_xCOR^a, (CH₂)_xCO₂R^a, (CH₂)_xCONR^aR^b, N═CHN(CH₃)₂ or (CH₂)_xCR^a═NOR^b, where x is 0 or 1, or said phenyl, pyridyl or thienyl may be substituted with (CH₂)_xAr³, COAr³ or CH(OH)Ar³ where Ar³ represents a five- or six-membered heteroaromatic ring optionally bearing up to 2 substituents selected from halogen, CN, CF₃, OH, C_1-6alkyl, C_1-6alkoxy, C_1-6alkylthio, amino, C_1-6alkylamino or di(C_1-6)alkylamino;

R^a and R^b independently represent H or a hydrocarbon group of up to 7 carbon atoms which is optionally substituted with up to 3 halogen atoms or with up to 2 substituents selected from CN, OH, C_1-4alkoxy, C_1-4alkylthio, amino, C_1-4alkylamino and di(C_1-4)alkylamino; or R^a or R^b, when linked through a nitrogen atom, together represent the residue of a heterocyclic ring of 4, 5 or 6 members, optionally bearing up to 3 substituents selected from halogen, CN, CF₃, oxo, OH, C_1-4alkyl or C_1-4alkoxy; or two R^a groups, when attached to adjacent carbon atoms of Ar², may form a fused ring of 5 or 6 members, 0-3 of which are selected from N, O or S while the remainder are carbon, said ring optionally bearing up to 3 substituents selected from halogen CN, CF₃, oxo, OH, C_1-4alkyl or C_1-4alkoxy;

and wherein any nitrogen atom forming part of a heteroaromatic ring may be in the form of the N-oxide;