Consider a series system consisting of sockets into each of which a component is inserted: if a component fails, it is replaced with a new identical one immediately and system operation resumes. An interesting question is: how to model the failure process of the system as a whole when the lifetime distribution of each component is unknown? This paper attempts to answer this question by developing two new models, for the cases of a specified and an unspecified number of sockets, respectively. It introduces the concept of a virtual component, and in this sense, we suppose that the effect of repair corresponds to replacement of the most reliable component in the system. It then discusses the probabilistic properties of the models and methods for parameter estimation. Based on six datasets of artificially generated system failures and a real-world dataset, the paper compares the performance of the proposed models with four other commonly used models: the renewal process, the geometric process, Kijima's generalised renewal process, and the power law process. The results show that the proposed models outperform these comparators on the datasets, based on the Akaike information criterion.Our responses are shown below, taking the reviewer comments one by one.In the revised paper, new text is coloured blue, deletions are indicated, and text we wish to emphasise to address a review comment is coloured red.Reviewer #1: This revised version significantly improves the first paper. I have still some comments that should be taken into account before acceptation.1. Throughout the paper, the components of the series systems are supposed to be independent. This assumption should be stated clearly. Response: We now state this clearly at the beginning of Proposition 1. Also, please note that the first sentence immediately under the heading of Section 2 says: "Consider a series system with a number of statistically independent components." (See the sentence in red on page 6).2. Answer to my comment 9. The proof (d) of the additivity of the CRIF is not really a proof. The additivity comes from the facts that the counting process of system failures is the sum of the counting processes of components failures, and that the probability of having more than one failure in an infinitesimal interval is zero because of the independence of the components. See considerations of that kind in reference 1. Response: Yes, we agree, and we have rewritten the text in part (d) there on page 8. In addition, the sentence starting with "Alternatively, …" shortly after is introduced to outline the idea of the proof and is in response to comment 1 of reviewer 3.3. Answer to my comment 15. You say that the sentence "and the oldest VC in the VSS will be renewed" has been removed. But in fact, it is still in the paper. You should explain this part more clearly. The fact that one component in the VSS is renewed seems contradictory with the initial assumption that the VSS is not maintained or minimally repaired. Response: We agree the explanation was ...