Questão jurídica principal
Whether the buyers' undertaking was a suretyship requiring public deed or a guarantee under Art. 111 CO
Decisão extraída
The undertaking was a guarantee under Art. 111 CO because it was independent of the principal debtor's duty and focused on the creditor's expected benefit.
Fundamentação extraída
The decisive criterion is accessoriness. The buyers promised supplementary contributions if the pension target was not reached, regardless of the employer's underlying obligation; the promise was creditor-oriented and independent, so it was not a suretyship.