Adamek and Sousa recently solved the problem of characterizing the subcategories K of a locally $\lambda$-presentable category C which are $\lambda$-orthogonal in C, using their concept of K$\lambda$-pure morphism. We strengthen the latter definition, in order to obtain a characterization of the classes defined by orthogonality with respect to $\lambda$-presentable morphisms (where $f : A \rightarrow B is called $\lambda$-presentable if it is a $\lambda$-presentable object of the comma category A/C). Those classes are natural examples of reflective subcategories defined by proper classes of morphisms. Adamek and Sousa's result follows from ours. We also prove that $\lambda$-presentable morphisms are precisely the pushouts of morphisms between $\lambda$-presentable objects of C.
Keywords: pure morphism, othogonality, injectivity, locally presentable categories, accessible categories
2000 MSC: 18A20, 18C35, 03C60, 18G05
Theory and Applications of Categories,
Vol. 12, 2004,
No. 12, pp 355-371.