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For a non-compact metrizable space $X$, let ${\mathcal E}(X)$ be the set ofall one-point metrizable extensions of $X$, and when $X$ is locally compact,let ${\mathcal E}_K(X)$ denote the set of all locally compact elements of${\mathcal E}(X)$ and $\lambda: {\mathcal E}(X)\rightarrow{\mathcal Z}(\betaX\backslash X)$ be the order-anti-isomorphism (onto its image) defined in:[HJW] M. Henriksen, L. Janos and R.G. Woods, Properties of one-pointcompletions of a non-compact metrizable space, Comment. Math. Univ. Carolinae46 (2005), 105-123. By definition $\lambda(Y)= \bigcap_{n<\omega}cl_{\betaX}(U_n\cap X)\backslash X$, where $Y=X\cup\{p\}\in{\mathcal E}(X)$ and$\{U_n\}_{n<\omega}$ is an open base at $p$ in $Y$. Answering the question of[HJW], we characterize the elements of the image of $\lambda$ as exactly thosenon-empty zero-sets of $\beta X$ which miss $X$, and the elements of the imageof ${\mathcal E}_K(X)$ under $\lambda $, as those which are moreover clopen in$\beta X\backslash X$. We then study the relation between ${\mathcal E}(X)$ and${\mathcal E}_K(X)$ and their order structures, and introduce a subset${\mathcal E}_S(X)$ of ${\mathcal E}(X)$. We conclude with some theorems on thecardinality of the sets ${\mathcal E}(X)$ and ${\mathcal E}_K(X)$, and someopen questions.

On one-point metrizable extensions of locally compact metrizable spaces